Ensemble River Forecasts and Flood Inundation Mapping - Maximizing Available IDSS Tools

The National Weather Service (NWS) has begun a multi-year effort to introduce Flood Inundation Mapping (FIM) services nationwide. The first FIM services were rolled out publicly to 10% of the US population in September 2023 with much more of the population scheduled to come online in the coming years. This presentation highlights the development of an ArcGIS Online (AGOL) FIM dashboard using ESRI Geographic Information System (GIS) software for data visualization. The goals of the dashboard are to a) combine necessary meteorologic and hydrologic layers for efficient FIM Impact Based Decision Services (IDSS), b) visualize river forecast ensemble information to highlight critical FIM locations, and c) streamline coordination between NWS field offices.

FIM services leverage the Height Above Nearest Drainage (HAND) method to produce flood extents based on the National Water Model (NWM) Latest Analysis, 5 Day Streamflow Guidance, and River Forecast Center streamflow forecasts. A robust training program was established for forecasts addressing the science of HAND, FIM services, and FIM IDSS. In NWS Eastern Region (ER), the participating 10% included four Weather Forecast Offices (WFOs) and three River Forecast Centers (RFCs). Prior to the public release of FIM, operational forecasters were specifically trained on the science and methodology behind FIM development, FIM data inputs, potential FIM limitations, how to conduct FIM reviews, and how to provide effective IDSS to partners.

Forecasters have limited time to complete FIM reviews during real-time events, so tools were needed to quickly identify key FIM areas for different time frames. To fill this need, an ER FIM Dashboard was developed for operational forecasters to quickly review and identify important FIM products for the near-term (0 to 2 day), short-term (5 day), and outlook (7-10 day) timeframes. The outlook period (7-10 day) dashboard tab in particular benefitted from the inclusion of the NWS Hydrologic Ensemble Forecast Service (HEFS) and Meteorological Model-based Ensemble River Forecasts (MMEFS) layers. These ensemble-based forecast products quickly identify potentially high river levels up to ten days in advance which can allow forecasters to review FIM accuracy with several days lead time and facilitate meaningful IDSS to begin well before an impactful event.

This presentation demonstrates how the NWS developed and leveraged the ER FIM Dashboard to enhance situational awareness of potential FIM locations using probabilistic ensemble-based river forecasts. The ER FIM Dashboard provides targeted visualizations of potentially impacted areas, leverages ensemble river forecasts, and enhances IDSS coordination across the agency.